I'm not sure about that, possibly he put some sort of special make-up/glue-ish stuff on his face to try to seal it in, and his clothes caught the rest of it? He

I'm not sure about that, possibly he put some sort of special make-up/glue-ish stuff on his face to try to seal it in, and his clothes caught the rest of it? He's seen putting stuff in his hair, probably for that purpose. Anyway, how sensitive are detectors in real life? Remember, you need enough intact genetic material to figure out who it is, it's not like detecting anthrax or something where you can just look for a few short conserved snippets. I imagine it's not something PCR works especially well on, so you might not be able to amplify it at all.